Constructing a building with improved energy efficiency often involves reducing the amount of air leaking between the inside and outside of the building. In the summer, infiltrating hot air increases cooling costs, in the winter, infiltrating cold air increases heating costs.
Unfortunately, buildings with substantially reduced air leakage may have undesirably high levels of indoor air pollution. Concentrations of carbon dioxide (from occupant respiration) formaldehyde (from building materials), radon, carbon monoxide, and vapors from cleaning solvents and the like, can rise to unacceptable levels if the building is not properly ventilated.
This apparent dilemma can be resolved with the use of an air-to-air heat exchanger which can increase ventilation without undoing the energy efficiency gains provided by building a "tight" structure. With such a heat exchanger, air exhausted from the building flows in close thermal proximity to incoming fresh air, separated only by the walls of the heat exchanger air conduits. Thus, in the winter, the heat from the warm air being exhausted is transferred, in part, to the cool air drawn in from the outside reducing the net heat loss. In the summer, the heat from the warm air being drawn in from the outside is, in part, transferred to the cool air being exhausted, reducing the net heat load on the air conditioning system.
The efficiency of an air-to-air heat exchanger depends in part on the amount of surface area shared between conduits of incoming and outgoing streams of air. Larger surface area increases the amount of heat flow between the opposed air streams. For this reason, a well designed heat exchanger core will separate the incoming and outgoing air streams into many alternating and interwoven channels to increase the surface area shared between the streams.
A "cross flow" heat exchanger core is generally block shaped comprised of many horizontal conductive plates dividing the air flow into many layers. Outgoing air passes along even layers at right angles to incoming air passing along odd layers. The perpendicular air flows help in segregating flow streams before and after they pass through the core. Such air-to-air heat exchanger cores are manufactured in various sizes by a number of manufacturers. Some core designs divide the air streams with heat conductive plates that are also water permeable to permit for not only heat exchange but humidity exchange.
It is important that the size of a heat exchanger, and in particular, the heat exchanger core, be properly matched to the required air flow and the expected air temperature and humidity conditions. Normally this is accomplished by selecting from among a variety of different pre-manufactured sizes of heat exchangers. Each such heat exchanger includes a different core size and/or combinations of cores. Because of the crossing intake and exhaust air streams which must be channeled separately to the heat exchanger core, different sizes of heat exchanger cores normally require a matching enclosure, unique to that core, so as to maintain the proper air flow and separation.
Manufacturing and stocking many different heat exchanger models is expensive and difficult.